Multi-leaf collimators and operating method

ABSTRACT

Multi-leaf collimators have a guide frame ( 1 ) with a plurality of metal plates ( 3 ) arranged in a displaceable fashion, by which each individual metal plate can be displaced by an electric motor (M), with the electric motor (M) being a rotary electromechanical motor (M), which operates according to the form-fit principle, with electromechanical actuators.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2007/060142 filed Sep. 25, 2007, which designatesthe United States of America, and claims priority to EP Application No.07018232.4 filed Sep. 17, 2007, the contents of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to multi-leaf collimators (MLC)/multi-platecollimators, which are functionally-definitive elements in large devicesfor radiation therapy.

BACKGROUND

The high-energy radiation generated by an x-ray source, a linearaccelerator for instance, such as gamma, x-ray or photon radiation, isshielded by an adjustable diaphragm system, which generally consists ofwolfram plates and the beam cross-section is formed, so-called “beamshaping” such that a target area, like a tumor for instance, is exposedto a maximum amount of radiation and surrounding healthy tissue isexposed to a minimum amount of radiation. To achieve the best possibleadjustment of the beam cross-section to the target area, the multi-leafcollimator consists of a plurality, for instance several hundred,adjustable thin individual plates. The radiation path in a radiationtherapy device consists of a high-energy radiation source, whichgenerates and emits high-energy radiation, a linear accelerator forinstance. One first simple electrically adjustable XY diaphragm systemlimits the radiation path such that the adjacent multi-leaf collimatorin the radiation path is fully illuminated. The multi-leaf collimatorthen structures the beam cross-section such that a preciselypredetermined region is radiated.

The optimization problem within radiation therapy consists in minimizingthe radiation dose, to which the healthy tissue is exposed, and in atleast maintaining it to below a harmful threshold and in simultaneouslyexposing cancerous tissue to a significantly harmful radiation dose. Themethods for radiation treatment are thus very different and areundergoing constant development. Examples worth mentioning here are:

-   -   Conformal Radio Therapy (CRT),    -   Intensity-Modulated Radiation Therapy (IMRT),    -   Image-Guided Radiation Therapy (IGRT) Gated treatments,    -   High-precision radiation therapy and radiation surgery        (SRT/SRS),    -   Future advanced adaptive therapies, such as Dose-Guided        Radiation Therapy (DGRT), as they become available.

The objectives here are to increase selectivity, expand the applicationbandwidths, such as radiating moving target areas for instance,increasing the operating reliability, such as increasing/extending theservice intervals and shortening the treatment duration, such as forinstance by “sliding window”. In particular, the latter method not onlyreduces the radiation exposure of the healthy tissue, but alsoinfluences the workflow and efficiency of the large devices. Thefollowing profile of requirements results herefrom for multi-leafcollimators:

-   -   High positioning accuracy of the wolfram plates (previously type        0.1 mm),    -   High movement speed of the wolfram plates (previously type 18        mm/s),    -   High acceleration of the wolfram plates (previously type 38        mm/s²),    -   High operating reliability/maintenance intervals (life-cycle        costs).

DC motors with front-sided planetary gears are currently used asmulti-leaf drives with a reduction of 1:275 for instance and a torque of0.84 Nm at a maximum of 0.44 rps, said DC motors being arranged ingroups of type 40 motors and driving the wolfram diaphragms overslanted-toothed helical pinions. Two linear potentiometers are presentper diaphragm in order to control and monitor the position. Thepositioning accuracy amounts to 0.5 mm in the isocenter, whichcorresponds to a control accuracy of the plates of 0.25 mm.

The prior art consists in radiating cancerous tissue from differentspatial directions, with the so-called “step and shoot” strategy beingused. The system is thus paused for each new adjustment. In this way, aspatial direction is displaced, the multi-leaf collimator is set up togenerate the optimum beam cross-section, and radiates according to apreviously determined radiation dose, the next position is displaced andthe multi-leaf collimator is set up again etc. A very long treatmentduration results due to the added setup times for the individualilluminations.

The aim is to radiate continuously using a rotating gantry/frame and adynamically variable multi-leaf collimator with a “sliding-window”. Fordevices of the next generation, higher adjustment speeds of more than 20mm/s are aimed at while simultaneously improving plate positioningaccuracy by more than 0.10 mm. These requirements are not restricted oronly to a minimum degree using current drive technology and can becostly to display.

The problem involved with using electrical motors for displacing themetal plates consists in the high moment of inertia of the rotor, thehigh rotor speed and thus a high rotation energy, thereby resulting inpoor dynamic characteristics. For this reason, braking or reversing thedirection of movement of the metal plates is associated with relativelylarge delay times. In order to reduce the high rotor speed from type10,000 rpm to a typical output speed of 60 rpm, electric motors alsorequire a multi-stage gear. As a result of the unavoidable gearboxclearance, the positioning accuracy of the output is restricted, also infact when an additional sensor is used for position detection purposes.

SUMMARY

According to various embodiments, a multi-leaf collimator can beprovided with essentially increased positioning accuracy as known in theprior art.

According to an embodiment, a multi-leaf collimator may comprise a guideframe with a plurality of metal plates arranged in a displaceablefashion, by means of which each individual metal plate can be displacedusing an electric motor, wherein the electrical motor is a rotatoryelectromechanical motor, operating according to the form-fit principle,with electromechanical actuators.

According to a further embodiment, the piezoelectric actuators can beselected from the group of electromechanical actuators consisting ofpiezoelectric actuators, electrostrictive actuators and magnetostrictiveactuators. According to a further embodiment, the piezoelectric motormay comprise at least two electromechanical piezoactuators and aninternally toothed driving ring, which can be excited by a stroke of theelectromechanical actuators to a circulating displacement movement andan externally toothed shaft which can be attached to the driving ring,so that the shaft can be rotated by means of the displacement movementof the driving ring. According to a further embodiment, a number ofpiezo-electric actuators, drive rings and shafts can be arranged in amotor housing. According to a further embodiment, the shaft of thepiezoelectric motor and the metal plates may comprise intermeshingtoothing systems, so that the rotation of the shaft can be convertedinto a linear movement of the metal plates. According to a furtherembodiment, the positioning of the metal plates can be controlledelectrically. According to a further embodiment, each metal plate can bemechanically coupled to at least one electrical linear transducer forposition monitoring purposes. According to a further embodiment, thepositioning of the metal plates can be electrically controlled, with asignal of the at least one electrical linear transducer of each metalplate being used as a control signal. According to a further embodiment,a control electronics system may be remote from the piezoelectric enginesuch that it is arranged in a region with a radiation dose which islower compared to that of the piezoelectric motor.

According to another embodiment, in a method for operating a multi-leafcollimator as for example described above, charging signals of thepiezoelectric actuators of the piezoelectric motors can be used forfunction monitoring purposes.

According to another embodiment, in a method for operating a multi-leafcollimator as for example described above, metal plates can be movedboth individually as well as simultaneously according to individualmovement profiles.

According to another embodiment, in a method for operating a multi-leafcollimator as for example described above, piezoactuators can bearranged at right angles to one another and operate according to thelongitudinal effect and can be controlled in each instance using asine/cosine voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated below with reference to schematicappending figures, in which;

FIG. 1 shows the guide frame 1 of a multi-leaf collimator, with aplurality of guide grooves 2 for receiving metal plates 3,

FIG. 2 shows a form-fit operating piezoelectric rotary motor M,schematically and as a real model,

FIG. 3 shows toothing, also helical teeth,

FIG. 4 shows the finished design of a multi-leaf collimator consistingof the guide frame 1, a plurality of guides 2 and metal plates 3 andmotors M,

FIG. 5 shows the complete design of a multi-leaf collimator consistingof the mirror-inverted arrangement of two constructions shown in FIG. 4,

FIG. 6 shows a controlling computer, which controls by way of theinformation path 10.

DETAILED DESCRIPTION

A form-fit operating electromechanical rotary motor M according to FIG.2 has a lower moment of inertia and less stored rotation energy.

It is advantageous to embody an electromechanical motor shown in FIG. 2as a piezomotor M, as a result of which it is possible to achieve veryrapid changes in movement such as stopping, accelerating, reversing thedirection of movement.

As a result of the absent gear and the form-fit power transmission, bymeans of micro toothing between the driving ring and the motor shaft 6,such a motor achieves very high positioning accuracy, without requiringa linear transducer therefor.

With the motor according to FIG. 2, a sine and cosine voltage areapplied in each instance to the at least two piezo actuators 7, 8arranged at right angles to one another and operating according to thelongitudinal effect in order to generate a wave rotation. The preciseposition of the motor shaft 6 is a function of the absolute phase of thesine and cosine drive voltage. The position can thus be extremelyaccurately controlled with very little electronic outlay and can beaccurately displaced both statistically and also dynamically at anytime.

FIG. 1 shows the guide frame 1 of a multi-leaf collimator, having aplurality of guide grooves 2 for receiving metal plates 3. The metalplates feature toothing 4 on a guide surface located in the direction ofmovement, into which toothing 4 an electrical drive can engage in orderto displace the metal plates in the guide frame.

A form-fit operating piezoelectric rotatory motor M according to FIG. 2has a low moment of inertia and little stored rotation energy. Thepiezomotor M shown in FIG. 2 thus enables very rapid changes in movementsuch as stopping, accelerating, reversing the direction of movement.

As a result of the absent gears and the form-fit power transmission bymeans of micro toothing between the driving ring 5 and motor shaft 6,such a motor achieves very high positioning accuracy, without requiringa linear transducer herefor.

With the motor according to FIG. 2, a sine and cosine voltage areapplied in each instance to the at least two piezo actuators 7, 8arranged at right angles to one another and operating according to thelongitudinal effect in order to generate a shaft rotation. The drivingring 5 is herewith moved in a circular fashion, with the motor shaft 6rolling along the inner surface of the driving ring 5 in a form-fitfashion.

The precise position of the motor shaft 6 is a function of the absolutephase of the sine and cosine drive voltage. The position can thus beextremely accurately controlled with very little electronic outlay andcan be accurately displaced both statistically and also dynamically atany time.

The conversion of the motor shaft rotation 6 into a linear movement ofthe metal plate 3 is carried out by a toothed wheel 9 which is fastenedto the motor shaft, said toothed wheel engaging into the linear toothingof the metal plate, see FIG. 3. In order to optimize the spaceavailable, the corresponding toothing can also be helical teeth, asshown for instance in FIG. 3 to FIG. 5.

FIG. 4 shows the finished design of a multi-leaf collimator of this typeconsisting of the guide frame 1, a plurality of guides 2, metal plates 3and piezo motors M, with a piezomotor driving a metal plate 3 in eachinstance. The piezo motors M, as shown in FIG. 4, are directly fastenedto the guide frame 1 in order to receive the reactive forces. They canhowever also be fastened to a carrier frame and preassembled as acomplete subsystem, which is then, on its part, connected to the guideframe or the housing of the multi-leaf collimator.

FIG. 5 shows the complete design of a multi-leaf collimator—consistingof the mirror-inverted arrangement of two constructions shown in FIG. 4.The multi-leaf collimator shown in FIG. 5 enables, in a window with thediameters A×B, a shaping of the beam cross-section with a transverseresolution according to the width of the metal plates and a longitudinalresolution according to the positional accuracy of the metal plates.

The high absolute accuracy of the form-fit piezomotors renders acomplicated control process superfluous, as a result of which thecontrol problem is reduced to purely motor control.

The reference character 13 illustrates the controlling computer in FIG.6, said computer receiving the data of the target area via theinformation path 10, according to the treatment plan, and receiving thedelay angle of the radiation source via the information path 12. Thecomputer 13 uses this data to calculate the optimum position of theindividual metal plates in order to adjust the beam contour. The targetvalue for each metal plate and thus for each of the piezomotors M issent from the computer 13, via a data bus, to the motor controller 14 indigital or analog form. Each motor controller 14 uses the target valueto calculate the necessary analogue control signal for the correspondingpiezomotor M. The analog motor control signal is transmitted from themotor controller 14 to the line driver 15, which provides the controlpower needed to drive the piezomotors M. The analog motor controlsignals are transmitted to the piezomotors M in the multi-leafcollimator by way of lines of length L. As there is no regulation loopand the motor control signals exhibit a high power, in the case ofpiezoelectric actuators in particular a high voltage amplitude, theconnecting line between the piezomotors M and the components of thecontrol electronics 13, 14, 15 can be designed to be comparatively long.The electronics system can herewith be positioned at a great distancefrom the radiation source, thereby significantly increasingservice-life, reliability and service intervals.

A multi-leaf collimator can be provided on each metal plate 3 in orderto monitor the position using at least one electrical linear transducer,such as a linear potentiometer for instance, which is mechanicallycoupled.

1. A multi-leaf collimator comprising: a guide frame, a plurality ofmetal plates arranged in the guide frame in a displaceable fashion, aplurality of piezoelectric motors, each piezoelectric motor including:at least two piezoelectric actuators, an internally toothed driving ringthat is excited by a stroke of the at least two piezoelectric actuatorsto a circulating displacement movement, and an externally toothed shaftattached to the driving ring such that the shaft is rotated by thedisplacement movement of the driving ring.
 2. The multi-leaf collimatoraccording to claim 1, comprising a motor housing that houses a number ofthe piezo-electric actuators, drive rings and shafts.
 3. The multi-leafcollimator according to claim 1, wherein a toothed coupling between eachexternally toothed shaft and a toothed portion of a corresponding metalplate converts a rotation of the shaft into a linear movement of themetal plate.
 4. The multi-leaf collimator according to claim 1, whereinthe positioning of the metal plates can be controlled electrically. 5.The multi-leaf collimator according to claim 1, comprising electricallinear transducers for monitoring the position of each metal plate. 6.The multi-leaf collimator according to claim 5, wherein the positioningof the metal plates is electrically controlled, with a signal of theelectrical linear transducer of each metal plate being used as a controlsignal.
 7. The multi-leaf collimator according to claim 1, wherein acontrol electronics system is remote from the electrical motor such thatit is arranged in a region with a radiation dose which is lower comparedto that of the electrical motor.
 8. A method for operating a multi-leafcollimator comprising a guide frame with a plurality of metal platesarranged in a displaceable fashion, and a plurality of piezoelectricmotors, each piezoelectric motor including at least two piezoelectricactuators, an internally toothed driving ring that is excited by astroke of the at least two piezoelectric actuators to a circulatordisplacement movement, and an externally toothed shaft attached to thedriving ring such that the shaft is rotated by the displacement movementof the driving ring, the method comprising: controlling the plurality ofpiezoelectric motors such that at least one of the metal plates ismovable both individually and simultaneously according to individualmovement profiles.
 9. A method for operating a multi-leaf collimatorcomprising a guide frame with a plurality of metal plates arranged in adisplaceable fashion, and a plurality of piezoelectric motors, eachpiezoelectric motor including at least two piezoelectric actuators, aninternally toothed driving ring that is excited by a stroke of the atleast two piezoelectric actuators to a circulating displacementmovement, and an externally toothed motor shaft attached to the drivingring such that the motor shaft is rotated by the displacement movementof the driving ring, the method comprising: for each piezoelectricmotor, controlling the at least two piezoelectric actuators using asine/cosine voltage to control the position of the motor shaft, whereinthe at least two piezoelectric actuators are arranged at right angles toone another and operate according to a longitudinal effect.